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Altostratus and altocumulus are different from one another in the same way that
cirrus and cirrostratus clouds are different from cirrocumulus clouds; in altocumu-
lus clouds, droplets predominate, giving them a crisp, sharper-edged look. With
altostratus clouds, ice crystals and snowflakes dominate or comprise the entire
cloud, giving it a diffuse, fibrous look. The observation of altocumulus and alto-
stratus clouds in the sky has long been a marker for deter iorating weather in the
hours ahead.
In spite of its name, altocumulus clouds are generally rather flat clouds that
strongly resemble stratocumulus clouds. An exception to this overall laminar archi-
tecture is in those species of altocumulus called ca stellanus and floccus. In these
forms, altocumulus clouds resemble miniature, lofted cumulus clouds that usually
occur in rows or patches rather than in widespread layers.
Altocumulus clouds are distinguished from cirrocumulus because they are lower
and the cloud elements in altocumulus are, or appear to be from the ground, several
times larger than those in cirrocumulus clouds. For example, the elements of an
altocumulus cloud are typically the wi dth of three fingers held skyward at the
ground. Also, shading toward the center of the thicker elements is usually present
in altocumulus clouds, a property that is not allowed in the classification of cirro-
cumulus clouds. Altocumulus clouds are distinguished from stratocumulus because
they are higher above ground level than stratocumulus (at least 2 km) and because
the individual cloud elements in altocum ulus are, or appear to be from the ground,
smaller than those in stratocumulus.
In spite of the gray shading that may be present, altocumulus clouds are rarely
more than 1 km thick. This is because the concentrations of drops in them are
relatively high (typically 50,000 or more per liter) compared with fibrous ice
clouds whose particle concentrations may only be only tens to a few thousand per
liter. This density of particles produces an optical depth in which the sun’s disk
can be obscured (optical depth of 4 or more) by an alto cumulus cloud layer only
300–500 m thick.
Altocumulus clouds sometimes sport patchy ‘‘virga.’’ Virga is light precipitation
that falls from a cloud but does not reach the ground because it evaporates. Because
virga is almost always caused by falling snow, it appears fibrous, often with striations
or long filaments that often far surpass the depth of the cloud from which it is falling.
Virga, because it is comprised of falling snow, can appear to be quite dense. Alto-
cumulus clouds with virga are predominantly those clouds whose temperatures are
lower than 10
C (Heymsfield, 1993). However, at the same time, they are rarely
colder than about 30
C. This is because at very low temperatures they are likely to
take on the attributes of ice clouds such as cirrus or its thicker brethren, altostratus.
The species of altocumulus clouds called altocumulus castellanus has always had
a special significance in meteorology because these clouds reveal a conditionally
unstable lapse rate in the middle troposphere. Instability of this sort has been viewed
as a marker for likely releases of deeper convection in the hours ahead. Occasionally,
castellanus clouds group and enlarge into higher based cumulus and cumulonimbus
clouds.
396 THE CLASSIFICATION OF CLOUDS
When the winds are relatively strong aloft (greater than about 20 m s
1
) and
moderately moist, but stable lapse rate conditions are present, a species of altocu-
mulus called lenticularis (lens or almond-shaped) clouds (Fig. 5) may form over or
downwind of mountains. Altocumulus lenticularis clouds can hover over the same
location for minutes to hours while expanding and shrinking in response to fluctua-
tions in the relative humidity of the air mass being lifted over the terrain. Because the
conditions under which these clouds form are most often associated with advancing
short wave troughs in the middle and upper atmosphere and their accompanying
regions of low pressure, lenticularis clouds are usually precursors to deteriorating
weather.
With altostratus clouds (Fig. 6), the dominance of ice causes a diffuse, amor-
phous appearance with striations or fallstreaks (virga) on the bottom; an observer is
usually viewing relatively low concentrations of precipitation particles rather than a
cloud per se. Altostratus clouds are rarely less than 2 km thick and often have tops at
the same heights as cirrus and cirrostratus clouds. Because of this great altitude
range, they are considerably colder and span a much greater temperature range than
do altocumulus clouds. They also, by definition, cover the entire sky or at least wide
portions of it; they are not patchy clouds. Precipitation is usually imminent when
altostratus clouds are moving in because they are a sign that a wi despread slab of air
is being lifted, usually that due to an approaching cyclone and its frontal system
(Brooks, 1951).
The relatively low concentrations of large particles in some alto stratus clouds
(often tens per liter) can allow the sun’s position to be seen as though looking
through ground or fogged glass: the sun’s position is apparent, but the outline of
its disk is not. This may be the case (translucidus variety) even when the cloud layer
is two or more kilometers thick.
Somewhat surprisingly, when the top of an altostratus cloud layer is warmer than
about 30 to 35
C, it is not uncommon to find that the top is comprised of a thin
droplet cloud virtually identical to an altocumulus cloud layer that is producing
virga. The survival, growth, aggregation, and breakup of ice crystals spawned by
these cold, liquid clouds over a great depth usually obscures the ice-producing
droplet cloud top in these cases. These kinds of situations were dubbed ‘‘the
upside-down storm’’ when first noticed in the mid-1950s because the coldest part
of the cloud (the top) was liquid and the warmer regions below were comprised of
ice (Cunningham, 1957).
Optical phenomena seen from the ground with altostratus clouds are limited to
parhelia (‘‘sun dogs’’). They are usually observed in thin portions of altostratus
when the sun is low in the sky. Parhelia are brigh t, colored highlights, which some-
times rival the brightness of the sun, that are located 22
from the sun’s position.
They are most noticeable in the morning or before sunset.
However, since the composition of the uppermost regions of the deepest alto-
stratus clouds are virtually identical to cirriform ice clouds with simpler, smaller ice
crystals, haloes are often observed in the upp ermost portions of deep altostratus
clouds.
4 THE CLASSIFICATION OF CLOUDS 397
Nimbostratus clouds (Fig. 7) are virtually identical to altostratus clouds in their
composition except that their bases are usually perceived from the ground as lower
than in altostratus (from which it has usually derived due to a downward thicken-
ing). Therefore, they often appear somewhat darker than altostratus clouds and, by
definition, do not allow the sun to be seen through them. The perceived base of
nimbostratus is caused by snowflakes that are melting into raindrops. This apparent
base of the cloud is a result of the greater opacity of snow particles giving the
impression of a bottom or sharp increase in thickness of the cloud at the melting
level. Thus, the base of an amorphous, precipitating nimbostratus cloud might
be perceived at mid-levels on a day when the freezing level is high (above 2 km)
such as in southern latitudes or the tropics, or be perceived as low when the freezing
level is low as in northern latitudes in the winter. Generally, the bottom of
nimbostratus clouds is obscur ed by lower detached clouds such as stratus fractus,
or stratocumulus.
Nimbostratus clouds produce relatively steady precipitation that often continues
for hours at a time. They are not clouds responsible for passing showers with periods
of sun in between. The tops of dark, steadily precipitating nimbostratus clouds can
be as shallow as 2–3 km and even be above freezing in temperature, or they may
reach into the upper regions of the troposphere (to cirriform cloud levels) and be as
cold as 80
C. At elevations above the freezing level nimbo stratus is largely
composed of ice crystals and snowflakes, though embedded thin (supercoooled)
droplet cloud layers similar to altocumulus clouds are relatively common. Also,
similar to altostratus clouds, when the temperature at the top of nimbostratus
clouds is above about 30
to 35
C, a thin droplet cloud layer may be found in
which the first ice crystals form.
A broken-to-overcast layer of shallow stratus or stratocumulus clouds often
resides at the bottom of nimbostratus clouds. However, while usually not precipitat-
ing themselves, these lower cloud layers are important in enhancing the amount of
rain or snow that falls from nimbostratus clouds. This enhancement occurs because
of the accretion or riming of the relatively small cloud drops in the lower clouds by
the rapidly falling precipitation-si zed particles. This enhancement is especially
evident in hilly or mountainous regions. However, just the existence of lower
clouds even with drops too small to be collected by the faster falling precipitation
indicates enhanced precipitatio n at those locations c ompared with cloud-free loca-
tions. This is because where there are no lower clouds, the precipitation is likely to
be subject to a degree of evaporation, and the drops or snowflakes are slightly
smaller in comparison to those locations where clouds exist below the nimbostratus
and there is no evaporation through the depth of the cloud.
Cumulonimbus clouds (see below) may also be embedded in nimbostratus clouds.
The presence of such clouds within nimbostratus is evident by sudden gushes of
much heavier rain and sometimes lightning within a context of relatively steady rain.
398 THE CLASSIFICATION OF CLOUDS
4.3 Low Stratiform Clouds
Stratocumulus and stratus clouds (Figs. 8 and 9, respectively) are low-based, shallow
stratiform clouds, almost always less than 1 km thick. They are composed of droplets
unless the cloud top is cooler than about 5to10
C in which case ice crystals
may form (Hobbs and Rangno, 1985). Stratocumulus clouds differ from stratus
clouds because they have a rather lumpy appearance at cloud base with darker
and lighter regions due to embedded weak convection. Also, their bases tend to
be higher, and more irregular in height than those of stratus clouds. Stratus presents
a smoother, lower, more uniform sky than do stratocumulus clouds because the
internal convective overtu rning that produces lighter and darker regions is slight.
Drizzle (precipitation comprised of drops < 500-mm diameter that nearly float in the
air) is likely to form when the cloud droplet concentrations are lower than about
100 cm
3
. Therefore, drizzle is common from both stratus and stratocumulus clouds
at sea and along and inland of coastlines in onshore flow because in such situations
the air is clean and there are relatively few cloud condensation nuclei on which
droplets can form. However, recent measurements have also shown that drizzle and
light rain producing shallow clouds with low droplet concentrations are much more
common at inland locations in winter than was once believed (e.g. Huffman and
Norman, 1988). Since such clouds are often supercooled in winter, they pose a
severe potential for aircraft icing and freezing rain or drizzle at the ground.
Figure 8 Stratocumulus: note the uneven bases of this cloud layer. The darker regions show
where there is enhanced modest convection and higher liquid water content.
4 THE CLASSIFICATION OF CLOUDS 399
4.4 Convective Clouds
Cumulus and cumulonimbus clouds (Figs. 10, 11, 12, and 13, respectively) are
convective clouds created when the temperature decreases rather rapidly with
increasing height. Differential heating and converging air currents in this circum-
stance can therefore send plumes of warmer air skyward with relative ease. Convec-
tive clouds are limited in coverage compared with stratiform clouds and, except for
the anvil portions of cumulonimbus clouds, rarely cover the entire sky or do so only
for short periods. This coverage characteristic differentiates, for example, strato-
cumulus clouds with their linked cloud bases covering large portions of the sky,
and similar-sized cumu lus clouds that by definition must be relatively scattered into
isolated clouds or small clusters with large sky openings.
Cumulus clouds have a size spectrum of their own that ranges from cumulus
fractus, those first cloud shreds that appear at the top of the convective boundary
layer, to congestus size (more than about 2 km deep). Between these sizes are
cumulus humilis and cumulus mediocris clouds, clouds that range between about
1 and 2 km in depth, respectively. The tops of these larger clouds are marked by
sprouting portions called turrets that represent the growing and usually warmer parts
of the cloud. Individual turrets are generally one to a few kilometers wide, although
in strong storms individual turrets may coalesce into groups of many turrets to form
Figure 9 Stratus: the clouds intercept hills only a few hundred meters above sea level. Note
here the uniformity of this cloud layer compared with stratocumulus in Fig. 8.
400
THE CLASSIFICATION OF CLOUDS
Figure 10 In this busy sky, cumulus mediocris is at the center, with cumulus humilis in the
distance, and cumulus congestus on the horizon at left. Also present in this photo is
altocumulus translucidus (left center), cirrocumulus (top center), and cirrus uncinus (center
above cumulus and right center).
Figure 11 Cumulonimbus calvus: this rapidly changing and expanding cumulonimbus cloud
is in the short-lived calvus stage where fibrousness of the top is just beginning to be apparent.
No strong rainshaft has yet appeared below base, though the careful eye can see that
considerable ice has already formed aloft (right half of cloud). An intense rainshaft emerged
below cloud base a few minutes after this photograph was taken.
4 THE CLASSIFICATION OF CLOUDS 401
a large, tightly packed, and hard-appearing cauliflower mass that roils upward with
little turret differentiation.
Prior to reaching the cumulonimbus stage, cumulus clouds are therefore
composed of droplets and contain very few precipitation-sized particles. Precipita-
tion, however, usually begins to develop in cumulus congestus clouds if they are
more than about 3 km thick over land and about 1.5 to 2 km thick over the oceans
(Ludlum, 1980; Wallace and Hobbs, 1977). The precipitation that falls may be
caused by collisions with coalescence of cloud drops in the upper portions of the
cloud or it may be a result of the formation of ice particles in clouds with cooler
bases. However, in the winter, even small cumulus clouds with tops colder than
about 10 to 15
C can produce virga, snow flurries, or even accumulating
amounts of snow. These kinds of small, cold-based, and precipitating cumulus
clouds are found in winter in such locations as the Gre at Lakes of the United
States, off the east coasts of the continents, and over high mountains or desert
regions (Rangno and Hobbs, 1994).
Figure 12 Cumulus congestus, cumulonimbus calvus, and cumulonimbus capillatus. In this
line of convection north of Seattle, WA, (nonprecipitating) cumulus congestus clouds
comprise the left side of the line, while in the center a taller single tur ret has emerged and
reached the cumulonimbus calvus stage. The right third of the photograph shows a
cumulonimbus capillatus, the stage that follows the calvus stage. In the capillatus stage, in
this instance consisting of a conglomerate of turrets, the tops have lost their compact
cumuliform look and are clearly fibrous and icy. Strands of precipitation are falling below
the bases of both types of cumulonimbus clouds.
402
THE CLASSIFICATION OF CLOUDS
If significant precipitation begins to develop in deep cumulus clouds, they quickly
take on the visual attributes of cumulonimbus clouds (Figs. 11–13)—a strong preci-
pitation shaft is seen below cloud base with a cloud top that is soft, fibrous, fraying,
or wispy. The visual transition to a softer, fibrous appearance in the upper portion of
cumulus clouds is caused by the lowering of the concentrations of the particles from
hundreds of thousands per liter of relatively small cloud droplets (< 50-mm
diameter), to only tens to hundreds per liter of much larger (millimeter-sized) preci-
pitation-sized particles (rain drops or ice particles). These larger particles tend to fall
in filaments and help produce a striated appearance.
In the period while this transformation is taking place and the fibrousness is just
becoming visually apparent in the upper portions of a cumulus congestus cloud, the
cloud is entering a short-lived period of its lifecycle when it is referred to as a
cumulonimbus calvus (‘‘bald’’) cloud. At this same time, a concentrated precipitation
shaft may not be present yet or is just emerging below the cloud base (Fig. 11).
When the fibrousness of the upper portion of the cloud is fully apparent, the
cumulonimbus cloud has transitioned to a cumulonimbus capillatus (hair) in which
most or all of its upper portion consists of ice crystals and snowflakes (Figs. 12 and
13). In the tropics or in warm humid air masses, this visual transformation also
occurs but can be due solely to the evaporation of the smaller drops leaving drizzle
and raindrops rather than ice and snow in smaller cumulonimbus clouds. Cumulo-
Figure 13 Cumulonimbus capillatus incus: this well-known species of cumulonimbus cloud
also has a fibrous and icy top, but it is marked by a noticeable flattening there. Incus translates
to ‘‘anvil.’’ These types of cumulonimbus clouds suggest that the convection has spanned the
entire troposphere.
4 THE CLASSIFICATION OF CLOUDS 403
nimbus capillatus clouds span a wide range of depths, from miniature versions only
about 2 km deep in polar air masses over the oceans, to as much as 20 km in the most
severe thunderstorms in equatorial regions, eastern China in the summer, and the
plains and southeast regions of the United States. If a pronounced flattening of the
top develops into a spread ing anvil then the cloud has achieved the status of a
cumulonimbus capillatus incus (incus meaning ‘‘anvil’’).
Hail or graupel (soft hail) are usually found, if not at the ground, then aloft in
virtually all cumulonimbus clouds that reach above freezing level. Updrafts may
reach tens of meters per second in cumulus and cumu lonimbus clouds, particularly
in warm air masses. These updrafts lead to large amounts of condensation and liquid
water co ntent. Depe nding on how warm the cloud base is, the middle and upper
building portions of deep cumulus clouds might contain 1 to 5 g m
3
of condensed
water in the form of cloud droplets and raindrops. Supercooled water concentrations
of these magnitudes are sufficient to cause a buildup of about 1 cm or more of ice
buildup on an airframe for every one to two minutes in cloud. Therefore, they are
avoided by aircraft. Cumulonimbus clouds are the only clouds, by definition, that
produce lightning. If lightning is observed, the cloud type producing it is automa-
tically designated a cumulonimbus.
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